1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, and more particularly, to an image forming apparatus that irradiates light on a photosensitive member in order to remove charge from it.
2. Description of the Related Art
Conventionally, an image forming apparatus employing an electrophotographic method is an apparatus employing a method and having a configuration in which a first image bearing member is caused to form and bear a transferable image by an appropriate image forming process; the transferable image is transferred from the first image bearing member to a second image bearing member; and the first image bearing member is repeatedly used for successive image formation.
For example, an image forming apparatus such as a copying machine and a laser beam printer is basically provided with an electrophotographic photosensitive member, which is generally a rotary drum, as the first image bearing member and charging means for uniformly charging the surface of the rotating photosensitive member to a predetermined polarity and potential.
Moreover, the image forming apparatus is provided with image exposing means for forming an electrostatic latent image on a charge processing surface of the rotating photosensitive member, developing means for developing the electrostatic latent image as a toner image and transferring means for transferring the toner image from the surface of the photosensitive member to a transfer paper as the second image bearing member (recording material).
Moreover, the image forming apparatus is provided with fixing means for fixing the toner image transferred to the transfer paper as a permanently fixed image and photosensitive member cleaning means (cleaner) for eliminating transferred residual toner on the surface of the rotating photosensitive member after the toner image has been transferred to the transfer paper so as to clean the surface of the photosensitive member.
The transfer paper to which image fixing processing is applied by the fixing means is discharged as an image formed product (copy or print).
The surface of the photosensitive member cleaned by the cleaning means is served for image formation repeatedly.
As a photosensitive member to be used for a conventional image forming apparatus of the electrophotographic method or the electrostatic recording method, a selenium series photosensitive member, an amorphous silicon (hereinafter referred to as a-Si) photosensitive member, an organic photosensitive member and the like are put to practical use. It is known that, among the photosensitive members, the a-Si photosensitive member is particularly excellent in terms of stability and durability.
The a-Si photosensitive member tends to absorb moisture because the surface of the photosensitive member becomes sensitive to humidity due to influence of a corona product caused by ozone that is generated from a back charger having a surface of high hardness This becomes a cause of drift of charge on the surface of the photosensitive member and brings about degradation of an image quality called a smeared image.
In order to prevent such a smeared image, various methods are used such as a method of heating with a heater described in Japanese Utility Model Examined Publication No. Hei 1-34205, a method of eliminating a corona product by rubbing a surface of a photosensitive drum by a brush formed of a magnet roller and magnetic toner described in Japanese Patent Examined Publication No. Hei 2-38956, and a method of eliminating a corona product by rubbing a surface of a photosensitive member by an elastic roller as described in Japanese Patent Application Laid-open No. Sho 61-100780.
On the other hand, contact charging is being put to practical use instead of corona charging as a charging method.
Contact charging is a method for charging the photosensitive member as a member to be charged to a predetermined polarity and potential of the surface of the photosensitive member by applying a predetermined charging bias to a conductive charging member such as a roller (charging roller), a fur brush, a magnetic brush or a blade and the like, and has advantages in that less ozone is generated and electric power consumption is small compared with the corona charging device.
Two types of charging methods, a corona charging method and a contact injection charging method, are mixed in a charging mechanism (charging principle) of the contact charging type, and respective properties emerge depending on which one is dominant.
The corona charging method uses a discharge phenomenon, such as corona discharge that occurs in a micro space between a contact charging member and a member to be charged, to charge the member to be charged by a discharge product. The corona charging method still generates a small amount of ozone, although it is markedly less than that in the case of the corona charger.
The contact injection charging method is a method in which charge is directly injected in a member to be charged from a contact charging member, whereby the surface of the member to be charged is charged.
This is also referred to as direct charging or injection charging. Japanese Patent Application Laid-open No. Hei 6-3921 and the like proposes a method of performing contact injection charging by injecting charge into a trap level existing on a surface of a photosensitive member or charge holding member, such as conductive particles and the like of a charge injection layer, using a contact charging member such as a charging roller, a fur brush or a magnetic brush and the like, in which conductive magnetic particles are magnetically constrained. Since contact injection charging does not use the discharge phenomenon, only a portion where the member to be charged and the charging member contact each other is charged.
Therefore, it is desirable to make peripheral speed differences or moving directions opposite each other in the charging member and the member to be charged so as to have a sufficiently high possibility of contact between the member to be charged and the charging member in order to perform fine charging without charging unevenly.
In addition, contact injection charging is a low power and ozoneless charging method in which there is no threshold voltage for starting charging and a voltage required for charging is only for a desired surface potential of a photosensitive member.
As a member to be charged for which the contact injection charging can be used, it is necessary to provide a charging injection layer in which conductive particulates as a charge holding member are dispersed on a surface of a photosensitive layer, for example, in the case of an organic photosensitive member. However, in the case of an inorganic photosensitive member represented by an a-Si photosensitive member, many trap levels based on a defect of a crystal exist on its surface even if a charge injection layer is not specifically provided, whereby injected charge is held by the trap levels and the injection charging can be applied to it.
Since contact injection charging does not generate a discharge product at all, if an a-Si photosensitive member is combined with the contact injection charging, there is a large advantage in that a smeared image that is a drawback of the a-Si photosensitive member can be fundamentally solved.
Therefore, it is unnecessary to provide means for always heating the a-Si photosensitive member with a heater or rubbing the surface of the photosensitive member with magnetic particles or an elastic member, whereby saving of power consumption and simplification of an apparatus can be realized.
However, the a-Si photosensitive member has such a characteristic that, if a light irradiated region and a dark region are simultaneously charged, attenuation (dark attenuation) of a potential is extremely large in the light irradiated region compared with the dark region, and a light memory (after image phenomenon) tends to occur.
That is, an a-Si series photosensitive member has many dangling bonds, which turn into localized levels that tend to trap a part of light carriers and lower its running performance or lower a recombination possibility of optically generated carriers.
Therefore, in the image forming process, a part of the light carriers generated by exposure is released from the localized level simultaneously when an electric field is applied to the a-Si series photosensitive member at the time of charging in the next step and a difference of surface potentials of the a-Si series photosensitive member occurs between an exposing portion and an unexposed portion, which eventually emerges as a smeared image due to a light memory.
Therefore, it is generally practiced to perform equal exposure in a main electricity eliminating process, thereby making light carriers latent inside the a-Si series photosensitive member excessive and equal/uniform over the entire surface so as to eliminate the light memory.
Thus, the light memory can be eliminated more effectively by increasing an amount of main electricity eliminating light emitted from a main electricity eliminating light source or by making a wavelength of the main electricity eliminating light close to a peak of spectral sensitivity (approximately 600 to 700 nm) of the a-Si series photosensitive member.
In a-Si, dark attenuation due to thermally generated carriers cannot be neglected and a potential tends to fluctuate due to temperature as well. Thus, image deterioration due to environmental fluctuation is prevented by using a potential stabilization technique as described below in addition to temperature control of a photosensitive member.
U.S. Pat. No. 2,956,487 discloses a method of measuring a latent potential of a photosensitive member by a potential sensor or sensing an image density developed on an electrostatic latent image and feeding a sensing signal back to charging means and image forming and exposing means of the image forming process to control them such that a desired potential can be obtained, thereby stabilizing an electronic photographic image. Such an image stabilizing method is generally performed when a power source is input to an apparatus or an image forming process is started or completed.
However, the conventional technology as described above has the following problems. In the case of a photosensitive member having many localized levels working as an electron trap like the a-Si, since accumulation of light carriers due to main electricity eliminating light progresses after starting image formation, a phenomenon of decreasing potential occurs.
The light carriers which are trapped enter a steady state while the charging process is repeated, and a potential is gradually stabilized. However, while a potential is decreasing, since the image formation cannot enter a step of potential control, there is a problem in that a first copying time is naturally extended.
Further, a state in which a potential is stabilized means that a balance among generation and accumulation of light carriers by main electricity eliminating light and recombination of light carriers or extinction of light carriers by emitting carriers trapped by charging becomes steady. The state varies according to an amount of light, a wavelength or a charging voltage.
The present invention has been devised in view of the above and other drawbacks, and it is an object of the present invention to provide an image forming apparatus capable of making potential control executable immediately after starting copying and shortening a first copying time.
It is another object of the present invention to provide an image forming apparatus having a photosensitive member; charging means for charging the photosensitive member; exposing means for exposing the photosensitive member charged by the charging means in order to form a latent image on the photosensitive member; and light irradiating means for irradiating light on the photosensitive member in order to eliminate a light memory of the photosensitive member, wherein the light irradiating means irradiates the photosensitive member with an amount of light larger than a minimum amount of light for eliminating a light memory in at least a part of an irradiation process.
Other objects and features of the present invention will be apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.